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Re: Re: Cable loss



At 09:20 AM 5/1/2005 +0100, Jules G0NZO wrote:
>Les,
>
>If you tower mount your PA/preamps, then the cable loss really isn't that
>important.
>As long as the preamp gain is enough to overcome the losses and the power
>reaching the mast is enough to drive your PA, then all should be fine. Shack
>mounting of PA's is really just the hard way of doing it!
>I have about the same distance to run as you and I use RG213. That has 6db
>of loss on my system, so 40W drive from the radio gives 10W at the mast,
>perfect to drive my PA. |The mast happens to have a shed next to it, which
>is ideal for housing the amps.
>For 23cms, I intend using a mast mounted PA, requiring 50mW of drive, so I
>will be using a long length of RG58 or even RG174 to act as an attenuator
>after the transmit converter's 1.5W output!
>
>I recon my aproach is still valid for terestrial work, with QRO valve amps.
>Shed mounted amps and sensible coax feeds to the shack (as I will be doing
>when I finally get 23cms EME going).
>
>(This is probably a whole big can of worms I'm opening here!)
>
>Jules
>G0NZO

Jules,

Not at all.  What you describe is fairly common practise by mw operators
since loss at say 10-GHz over more than a couple feet is excessive unless a
long run of waveguide is used (difficult to run past rotators).  

I gave Les the same suggestion, as I currently do this for my roof-top
located satellite array.  I have a Hoffman box to contain my preamps,
converters, and PA's including an 8-amp PS to avoid long runs of No. 6 or
No. 8 dc wiring.  My cable run is only 50-feet but I can use cheap RG-58,
RG-59 (for the Drake s-band conv.), and RG-213 (RG-8) instead of LDF4 or
LDF5 hardline.

I would agree with most of your approach except high-power tube (valve)
amps which usually need frequency touch-up on tuning as they warm up in
transmit.  Also, if one uses water-cooled tube amps then in northern
climates one must heat the shelter to avoid freezing the cooling water
(antifreeze cannot be used with High-voltage since it leads to arc-over
thru the coolant to ground).  Likewise, I would not recommend long runs of
RG-174; instead use a low-power attenuator at the far end of RG-58.
73's,
Ed - KL7UW  <new call>
=================================
http://www.qsl.net/al7eb
144-MHz EME - BP40iq:
FT-847, mgf-1801/1402, 4xM2-xpol-20, 170w
=================================
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